Metering elements in the form of ink slides are already known for adjustment of an ink film thickness profile on the ink fountain roller in the inking unit of offset printing presses. The ink film thickness is an indication of the quantity of ink required during printing, which is determined by the distribution of the printing areas on the printing plate. It represents relatively uniform ink supply over the width of the printing plate in contrast to the control possibilities over the width or length of ink strips with constant thickness. The ink film thickness used for metering corresponds to the smallest available dimension. Correspondingly narrow tolerances should be selected in the adjusting mechanism in the case of high accuracy requirements of the settings. In addition to a favorable, preferably linear adjustment characteristics, a high degree of reproducibility for the existing adjusting mechanisms is also required. Furthermore, the setting should not be affected by interruptions in the metering edge or displacement between metering elements and ink fountain roller. Effects of this type should be avoided, if possible.
Approximately continuous adjustment over the ink fountain width can be achieved with a continuous ductor blade. However, insoluble problems occur in the adjustment as a result of discontinuities in the event of extreme differences in ink consumption.
Use of the surface of the ink fountain roller as a reference area for adjustment of the individual metering elements in relation to this surface requires supports, which interrupt the metering areas and lead to non-uniform ink supply, wearing parts and complicated construction of the metering elements. Hence the use of linearly adjustable metering elements of duct width on an ink fountain guided in relation to the ink fountain roller remains. According to DE-OS No. 3,218,045 the ink fountain can be guided on both sides by cam rollers on the surface of the ink fountain roller. The position of the metering elements in relation to the ink fountain roller will thus be clearly fixed with a high degree of reliability. The metering elements themselves must be guided with minimum play to ensure adequate reproducibility.
A metering element of this type is described in U.S. Pat. No. 2,583,640. An ink fountain is provided with ductor blade sections corresponding to the width of an ink duct. The ductor blade sections are arranged on a bottom part of the ink fountain so as to be movable longitudinally and disposed with their metering edge adjacent the surface of an ink fountain roller. The direction of movement of the metering edge intersects the cross-section of the ink fountain roller essentially as a tangent. The ductor blade sections are provided with threaded holes in their rear end and in their longitudinal direction. The threaded ends of adjusting pins are screwed into these threaded holes. The adjusting pins are rotatable at the other end, but held immovably in the longitudinal direction and provided with a handle. The ductor blade sections are protected against penetrating ink by a cover plate in the area of the adjusting pins. The cover plate is pulled flexibly from below against the ductor blade sections and has a support at the rear end towards the bottom part of the ink fountain and a rear wall, through which the adjusting pins are guided. Helical springs, which force the play out of the screwed part of the adjusting pins into the ductor blade sections and their holder on the ink fountain, are arranged between the support and the ductor blade sections. The ink fountain roller is adjustable on the ink fountain.
The entire arrangement of the ductor blade sections, however, has significant disadvantages. It necessarily leads to large inaccuracies in the adjustment of the metering gap between the metering edge and the ink fountain roller. The ductor blade sections are flexible in the longitudinal direction and their long sides also lie close together. Apart from sealing problems, larger friction forces, which must be absorbed at least by the helical springs to compensate for play, can be anticipated in the case of adequately tight packing. The adjusting forces thus increase automatically or the adjustment is no longer reproducible. On the other hand sufficiently straight guidance of the ductor blade sections cannot be ensured despite this arrangement.
Furthermore, heavy wear will inevitably result when the metering edge contacts the ink fountain roller, because either the strong springs act on the metering edge or rigid support via the adjusting pins is achieved when the play is overcome. The expansions cannot be equalized under the effect of heat, because the metering gap is reduced at right angles to the direction of movement of the ductor blade sections. Highly accurate adjustment of the metering edge in relation to the ink fountain roller is important and indeed critical when only a small ink supply or very small ink film thickness is required. This is markedly affected by the above-mentioned shortcomings. The arrangement of the ductor blade sections near a tangent to the circumference of the ink fountain roller and the resulting wedge-shaped gap also permit build-up of a high dynamic pressure in the printing ink, which additionally affects the accuracy of adjustment of the metering gap.